1. Field of the Invention
The present invention relates to a method and a device used to generate a digital tomosynthetic 3D X-ray image of an examined object, particular in mammography.
2. Description of the Prior Art
When an X-ray image of an examined object is being prepared, for example, for mammography, it must ensure that the radiographic parameters, particularly the exposure time, are set correctly so as to guarantee an image quality suitable for diagnostic evaluation. In order to limit the exposure of the area of the examined object that is being irradiated to the minimum radiation required for diagnostic purposes, it is desirable to be able to correctly set the radiographic parameters with only one attempt. For this purpose, a so-called automatic exposure control (AEC) is conventionally used. In analog mammography, as seen in the direction of propagation of the X-rays, a number of solid-state detectors are arranged behind the X-ray film. These detectors measure the intensity of the X-rays transmitted through the X-ray film and their output signals are then used to control the radiographic parameters (for example, exposure time, operation voltage of the X-ray tube, tube current, and the anode filter combination).
With continually improving imaging techniques, a goal is to generate X-ray images with a high diagnostic relevance using the smallest X-ray doses possible. For example, the imaging techniques must still be able to distinguish benign changes from malignant changes in mammography and should reduce the number of false findings, i.e., the number of suspect findings that are not caused by malignant changes, and the number of undiscovered malignant tumors. In the case of the conventional analog X-ray mammography, a two-dimensional single image of the compressed breast in a single projection direction is generated. In such a projection the subsequent tissue layers in the direction of the X-ray are superimposed, and thus strongly absorbing benign structures may cover a malignant tumor and make the identification thereof more difficult.
In order to avoid this situation and others like it, T. Wu et al., Tomographic mammography using a limited number of low-dose cone-beam projection images; Med. Phys. 30, 365 (2003) discloses a mammography method called tomosynthesis, by means of which individual images of the female breast are obtained at different projection directions using a digital X-ray detector. Using these individual digital images that have been obtained at different projection directions, or rather the image data pertaining to these individual images, a number of slice images are reconstructed that reproduce the layers of the breast that are oriented parallel to the reception surface of the X-ray detector. In the following, such image records obtained by reconstruction will be called tomosynthetic 3D X-ray images. These measures allow one to recognize the tissue structures to be recognized better than in a conventional single projection radiograph.
In X-ray tomosynthesis, however, it is not possible to attain a suitable degree of exposure control due to the higher absorption of the digital X-ray detectors used in tomosynthesis.